The present invention generally relates to an electrochemical cell. More particularly, the present invention relates to the mechanical construction of a battery.
Due to environmental concerns in recycling batteries, conventional alkaline batteries are now being manufactured without utilizing mercury. Although the elimination of mercury from an alkaline battery eliminates such environmental concerns, zero-mercury batteries tend to generate greater volumes of gas, which is primarily hydrogen, during the lifetime of the battery. This potential increase in gassing can cause several problems. First, sufficient space within the electrochemical cell must be provided for the accumulation of such gases. Because more space is required to accommodate the potential increased volume of generated gases in a zero-mercury battery, the amounts of active ingredients forming the anode and the cathode must be reduced. Such a reduction in active ingredients undesirably reduces the service life of the battery.
Another problem associated with the increased gassing in a zero-mercury battery is that the additional gases produced during discharge significantly increase the pressure inside the cell. To prevent the cells from bursting as a result of this increased pressure, pressure relief points are built in to the structure of the cell as a safeguard. An example of a conventional cell construction having such safeguards is described below with reference to FIG. 1.
As shown in FIG. 1, a conventional battery 10 includes a steel can 15 having an open end 16 and a closed end defined by a bottom surface 18. Battery 10 further includes a first electrode material 20 provided in electrical contact with the interior side surfaces and bottom surface 18 of can 15. A second electrode 25 is provided in the interior portion of battery 10 and is separated from first electrode 20 by a separator layer (not shown). After the electrode materials are inserted into can 15, a subassembly consisting of a seal 30 and a cover 40 is inserted into open end 16 of can 15. The inner cover 40, which is generally shaped as a disk having a central hole, is inserted such that its peripheral surface is in contact with an outer portion 31 of seal 30 and its surface defining the central hole is fitted around an inner hub portion 32 of seal 30.
As shown in FIG. 1, inner hub portion 32 of seal 30 includes a central hole 33 for receiving a current collector nail 45, which is driven through hole 33 thereby compressing the portions of seal 30 that are in contact with inner cover 40. After collector nail 45 is fully driven in place, a negative outer cover 50 is inserted in an open area defined by the outer portions 31 of seal 30. Negative cover 50 is provided in electrical contact with the head 46 of collector nail 45 while being electrically insulated from can 15 by seal 30. Once negative cover 50 is in place, the side walls of can 15 in the vicinity of opening 16 are crimped inwardly to secure negative cover 50 in place. A positive cover 60 is then welded to bottom surface 18 of can 15. Subsequently, a label 65, which may be formed of a shrinkable PVC material, is secured about the periphery of can 15.
As a safeguard against excessive internal pressure resulting from gas build-up, a pressure relief point 35 is provided in the lower portion of seal 30 by forming the seal with a relatively thin section that tears open when the internal cell pressure becomes excessive. To allow the gas to then vent to the outside of the battery, one or more vent holes 42 are provided through inner cover 40 and a plurality of vent holes 52 are provided in negative cover 50.
The primary functions of a seal are to prevent the materials inside the cell from leaking out while allowing hydrogen gas to pass through the seal and escape. By allowing the hydrogen gas to escape, the pressure level inside the cell is less likely to become excessive. Because the seals typically used in such alkaline batteries are made of a nylon or other resinous materials, an asphalt coating 38 is typically provided on high stress areas of the lower surface of seal 30 to prevent deterioration thereof resulting from contact with the active ingredients inside the cell. Although hydrogen can penetrate through the asphalt coating, the entire lower surface of seal 30 is typically not coated in order to allow an optimal amount of hydrogen to escape.